1. Field of the Invention
Embodiments of the invention relate to production of rare earth metals and/or metal mixtures from rare earth metal compound containing mixtures.
2. Description of Related Art
Rare earth metals, comprising metals of the series in the periodic table from lanthanum to lutetium, are very costly to extract from their respective ores. In large part, the cost is due to the large amount of waste, chiefly aqueous waste, that is generated by all stages of processing mined ore into mineral concentrate, leached concentrate, and the many intermediates between this and finished metal product. This very large volume of metal-contaminated aqueous waste renders prevention of emissions according to environmental regulations prohibitively costly. For these reasons, the Mountain Pass rare earth metal mine and processing facility in California, which is the largest such facility in the United States for decades, ceased its mining and processing operations in 1998, and only resumed in 2011.
Extraction of metals from their corresponding ores can be performed either by electrochemical or pyrometallurgical processes. The most commonly used method of pyrometallurgical process is smelting, wherein the ore is heated with a reducing agent to change the oxidation state of the metal ore and thereby generate the metal. Most ores are impure, thus requiring a flux, such as limestone, to combine with the byproducts and unreacted ore in order generate slag. Slag is subsequently removed to provide the refined metal.
The most commonly used method of electrochemical extraction is electrolysis, wherein the metal-containing ore is dissolved into a solution or melted to induce dissociation into its corresponding ionic components. Application of an electric potential across electrodes in the solution/melt induces reductive deposition of the metal at the cathode. Drawbacks of conventional electrolytic refining processes include decreased efficiency of refinement of metals with multiple oxidation states, which becomes increasingly relevant with respect to rare earth metals. Rare earth metals pose additional refinement and extraction challenges due to their very close electronegativities, which can complicate the electrochemical process.
Recent development of the solid oxide membrane (SOM) electrolysis process has provided an alternative electrochemical method for refinement of metal oxides (see, for example, U.S. Pat. Nos. 5,976,345 and 6,299,742). The SOM process comprises a solid oxygen ion-conducting membrane (SOM) typically consisting of zirconia stabilized by yttria (YSZ) or other low valence oxide-stabilized zirconia, for example, magnesia- or calcia-stabilized zirconia (MSZ or CSZ, respectively) in physical contact with the molten salt bath, an anode in ion-conducting contact with the solid oxygen ion-conducting membrane, and a power supply for establishing a potential between the cathode and anode. The metal cations are reduced to metal at the cathode, and oxygen ions migrate through the membrane to the anode where they are oxidized to produce oxygen gas. The SOM blocks ion cycling, which is the tendency for subvalent cations to be re-oxidized at the anode, by removing the connection between the anode and the metal ion containing molten salt. The SOM also protects and enables the use of a variety of oxygen-producing inert anodes to achieve high purity oxygen by-products and prevents back reaction (oxidation of the metal deposited at the cathode) via physical separation of the cathode product from the oxygen. The first demonstration of the SOM process produced a few tenths of a gram of iron and silicon in a steelmaking slag, and the process has made progress toward the production of other metals such as magnesium, tantalum and titanium (see, for example, U.S. Pat. No. 6,299,742; Pal and Powell, JOM 2007, 59(5):44-49; Metall. Trans. 31B:733, August 2000; Krishnan et al, Metall. Mater. Trans. 36B:463-473 (2005); and Krishnan et al, Scand. J. Metall. 34(5): 293-301 (2005)).
In the context of rare earth metals, extraction presents unique challenges, including very close electronegativities. Although reduction of rare earth metal oxides dissolved in molten salts has been demonstrated (see, Kaneko et al, J. Alloys & Compounds 1993, 193:44-46), commercial application of such processes remains prohibitive due to at least two reasons: 1) they require expensive pure rare earth metal oxides as a starting point, and 2) with multi-valent species such as cerium (which can exist in a 3+ or 4+ ion as well as a metal), electrolysis current efficiency is typically very low. SOM electrolysis in part overcomes these limitations by producing high-purity metals from moderate-purity oxides and by blocking ion cycling (see, WO/2010/126597). However, the SOM process requires the input of a relatively pure rare earth metal oxide or mixture of oxides in lieu of mineral ores that contain metal oxyfluorides. Prior to feeding into the SOM process, the naturally impure mineral ores must be processed to separate and refine the rare earth oxyfluorides to remove non-rare earth oxides such as calcium oxide or barium oxide, followed by conversion of the oxyfluorides to rare earth oxides. The rare earth oxides can then be fed into the SOM process.
Thus, there remains a need for more efficient and scalable apparatuses and processes to directly process rare earth metal oxide containing ore into pure metals.